Russia, with the largest territory in the world, has roughly the same total population as Nigeria, a country 1/16 its size.
俄罗斯是世界上版图面积最大的国家，拥有着几乎与尼日利亚相同的人口数量，而尼日利亚的国土面积仅是俄罗斯的1/16。
But this similarity won't last long.
但在不久的的将来两国人口数量将产生一定的差距。
One of the populations is rapidly growing, while the other is slowly declining.
其中一个国家的人口数量正在急剧增加，而另一个国家的人口数量在缓慢减少。
What can this tell us about the two countries?
我们能够从中得到关于这两个国家的哪些信息呢？
Population statistics are some of the most important data social scientists and policy experts have to work with.
人口统计数据是众多的社会科学家以及政策专家必须研究的重要数据之一。
But understanding a country's situation and making accurate predictions requires knowing not just the total size of the population
然而，要了解一个国家的国情并做出准确的预测，不仅需要知道其人口总数，
but its internal characteristics, such as age and gender distribution.
更要知道数据中所隐含的的一些内在特点，比如年龄和性别分布。
So, how can we keep track of all that data in a way that makes it easy to comprehend?
因此，我们如何用一种通俗易懂的方式来解读所有相关数据呢？
Complex data is more easily interpreted through visualization,
将复杂的数据可视化会使其更加简单易懂，
and one of the ways that demographers represent the internal distribution of a population is the population pyramid.
通常人口学家会用人口金字塔的方式来展现人口分布。
Here, the data is divided by gender with females on one side and males on the other.
在“人口金字塔”中，人口数据会按性别划分开，男女各占一边。
The population numbers are shown for each five-year age interval, starting from 0-4 and continuing up to 100 and up.
所显示的人口数量以五岁为一个年龄层，始于0-4，继续至100及以上。
These intervals are grouped together into pre-reproductive (0-14), reproductive (15-44), and post-reproductive years (45 and up).
这些年龄层被分成了：预生殖期阶段（0-14），生殖期（15-44）以及后生殖期（45及以上）。
Such a population pyramid can be a powerful predictor of future population trends.
这种人口金字塔可以成为预测未来人口发展趋势的有力工具。
For example, Rwanda's population pyramid shows it to be a fast-growing country,
比方说，卢旺达的人口金字塔显示，卢旺达是一个人口快速增长的国家，
with most of the population being in the youngest age groups at the bottom of the pyramid.
该国的大多数人口为金字塔底端的最年轻的群体。
The number will grow rapidly in the coming years.
在未来的几年里，该国的人口数量将会快速增长。
As today's children reach their reproductive years and have children of their own,
随着如今的孩子达到生育期并且有了他们自己的孩子，
the total population is almost certain to double within the next few decades.
总人口数量很有可能在将来的几十年里翻一翻。
For our second example, let's look at Canada, where most of the population is clustered around the middle of the graph.
再举一个例子，让我们来看看加拿大，该国的大多数人口聚集在图表的中间部分。
Because there are less people in the pre-reproductive age groups than there are in the reproductive ones,
因为处在预生殖期阶段的人口数量少于在生殖期阶段的人口数量，
the population will grow more slowly, as the number of people reaching their reproductive years decreases.
因此随着未来达到生殖期的人口数量的减少，该国的人口总数在未来将会增长的比较缓慢。
Finally, let's look at Japan. Because the majority of its population is in its post-reproductive years
最后，我们来看看日本。由于其大部分人口都处在后生殖期阶段，
and the number of people is smaller at each younger interval,
而年龄越小的阶段的人口越少，
this means that at current rates of reproduction the population will begin to decline as fewer and fewer people reach reproductive age.
这就意味着，按目前的生殖率计算，将来的人口总量将会下降，因为达到生殖年龄的人口将越来越少。

Comparing these three population pyramids side by side shows us three different stages in a demographic transition,
依次比较三种人口金字塔模型，表现出人口数量变化将要经历的三种不同阶段，
as a country moves from a pre-industrial society to one with an industrial or post-industrial economy.
这是一个国家从前工业化阶段进入到工业化或后工业化时期遇到的。
Countries that have only recently begun the process of industrialization
刚开始工业化进程的国家
typically see an increase in life expectancy and a fall in child mortality rates
通常会伴随着人口平均寿命的增长和儿童死亡率的下降，
as a result of improvements in medicine, sanitation, and food supply.
这是由于在医药、卫生和食物供给方面的改善。
While birth rates remain constant, leading to a population boom.
当出生率的达到稳定时，人口数量将迎来爆炸性增长。
Developing countries that are farther along in the industrialization process begin to see a fall in birth rates,
那些工业化更加发达的的发展中国家将出现生育率下降，
due to factors such as increased education and opportunities for women outside of child-rearing
原因在于其妇女在除了抚养孩子以外，受教育的程度和就业机会得到改善，
and a move from rural to urban living that makes having large families less economically advantageous.
以及从农村到城市的生活变迁使得孩子多的大家庭的生活经济成本增加。
Finally, countries in advanced stages of industrialization reach a point where both birth and death rates are low,
最后，那些处在工业化鼎盛时期的国家，其出生率和死亡率都比较小，
and the population remains stable or even begins to decline.
人口数量较稳定，甚至开始下降。
Now, let's take a look at the projected population pyramids for the same three countries in 2050.
现在，让我们来预测以上三个国家在2050年的人口金字塔模型。
What do these tell us about the expected changes in each country's population,
这些能够告诉我们每个国家将来人口数量变化的什么信息呢，
and what kinds of factors can alter the shape of these future pyramids?
以及哪些因素能够改变这些预测模型的形状？
A population pyramid can be useful not only as a predictor of a country's future but as a record of its past.
人口金字塔模型不仅可以用来预测一个国家的未来，同时也反映着它的历史。
Russia's population pyramid still bears the scars of World War II,
从俄罗斯的人口金字塔中我们仍然能够看到第二次世界大战的伤痕，
which explains both the fewer numbers of elderly men compared to elderly women
一方面因为老人中男性的数量少于女性，
and the relatively sudden population increase as soldiers returned from the war and normal life resumed.
另一方面可以看到男性的数量在之后突然猛增，这是因为士兵从战场上回来，回复到正常生活中。
China's population pyramid reflects the establishment of the one child policy 35 years before,
中国的人口金字塔反映了其计划生育政策实行了35年的效果，
which prevented a population boom such as that of Rwanda but also led to sex-selective abortions,
该政策避免了像卢旺达那样的人口爆发，但也导致了因人为选择性别而引发的堕胎，
resulting in more male children than female children.
其结果是男孩的数量多于女孩。
Finally, the pyramid for the United States shows the baby boom that followed World War II.
最后，美国的人口金字塔反映了第二次世界大战之后的生育高峰。
As you can see, population pyramids tell us far more about a country than just a set of numbers,
由此可见，人口金字塔能告诉我们的信息远不止一套数字，
by showing both where it's been and where it's headed within a single image.
仅仅通过一个图表，它能够告诉我们一个国家的过去和未来。
And in today's increasingly interconnected world,
在如今越来越紧密联系的世界中，
facing issues such as food shortages, ecological threats, and economic disparities,
面对像食物短缺、生态系统威胁和经济不平等诸多问题，
it is increasingly important for both scientists and policy makers
越来越重要的一点是，科学家和决策者们
to have a rich and complex understanding of populations and the factors affecting them.
能够认真和密切地认识到人口数量及其影响因素。